1. Field of the Invention
This invention relates to a liquid crystal display panel and more particularly pertains to an improved active matrix type liquid crystal display panel having switching elements such as a thin film transistor.
2. Description of the Prior Art
In recent years, liquid crystal display apparatuses have drawn great attention because they consume rather low electric power, the use of liquid crystal makes it possible to reduce their thickness and permits the manufacture of a large area display. Such liquid crystal display apparatuses are used, in a great extent, as the display means for wrist watch, electric calculator and other electric devices such as computers and TVs.
However, conventional twisted nematic type liquid crystal display apparatuses are incomplete in their display capacity and the rise of electric potential-contrast property. Then, in order to eliminate such disadvantages of the conventional liquid crystal display apparatus a matrix type apparatus has been developed, which is provided with non-linear elements or switching elements combined therewith. In this connection, a thin film transistor (TFT) or a diode which includes the use of amorphous silicon, polycrystalline silicon or a compound semiconductor or a varistor utilizing zinc oxide has been thoroughly examined.
Thus, a large number of matrix type display apparatuses have been developed having large areas, as means for displaying graphs, characters, images or the like. Among various kinds of flat panel display means, active matrix type liquid crystal display apparatuses have considerable potential because they permits the display of large capacities and large area and the reduction of electric power required to operate the apparatus.
Generally, a matrix liquid crystal display panel comprises an active matrix substrate provided with switching elements connected to scanning and signal lines, an electrode substrate faced thereto and a liquid crystal material filled in the space formed between these two substrates. More particularly, a typical structure of such an active matrix type display panel is disclosed in Appl. Phys., 1981, 24, 357-362, A. J. Smell, et al. and is composed of a glass substrate on which a plurality of thin film transistors (TFT: usually MOS type field effect transistor) as switching elements are mounted in a matrix form and which is covered with a transparent cover plate with liquid crystal injected between the glass substrate and the cover plate. The inner surface of the cover plate is coated with a transparent conducting layer of ITO (Indium-Tin-Oxide). The transmittance of the liquid crystal at respective picture elements is controlled by selectively driving the TFT's.
This active matrix type liquid crystal display panel makes it possible to eliminate the above mentioned disadvantages of the classical liquid crystal displays and presents an excellent rise of electric potential-contrast property and a high display capacity and permits a large area display. However, conventional active matrix panels have a large number of disadvantages such that the density of picture defects has not yet been reduced sufficiently. This is mainly due to the defects in at least one element such as switching elements, signal lines and scanning lines. This is because one picture element in the conventional display panel is composed of one switching element, one display electrode, one signal line and one scanning line.
The TFT's are, for example, formed using a polycrystalline silicon layer or an amorphous silicon layer. By use of the TFT's, the resolution of the display panel is improved to a practical extent, however, the density of picture defects has not been reduced sufficiently, since it is impossible, by the present production technique, to increase the yield of TFT's almost equal to one and therefore, the picture elements corresponding to defective TFT's appear as picture defects. For instance, if the yield of the TFT's is assumed to be 99.9 %, 250 picture defects appear on the display panel having 500 (rows) .times. 500 (columns) picture elements in a matrix. Such defective images cannot be practically used on the market. In addition, the high density of picture defects limits the application of the liquid crystal display panels only to small size displays.
In this respect, the applicant of this invention has developed a driving circuit for a liquid crystal display panel which permits the reduction of picture defects caused due to the low yield of TFT which is used as the switching element. The improved liquid crystal display panel comprises a plurality of scanning lines receiving scanning signals, a plurality of signal lines arranged orthogonally to the scanning lines and receiving driving signals, a plural sets of switching elements, each set being disposed at a crossing point of the scanning lines and the signal lines, and a plurality of liquid crystal elements coupled with the respective switching elements, characterized by having a plural number of switching elements per one picture element which is divided into a plurality of pieces (sub-elements), each piece having a display electrode coupled with one switching element, the switching elements being switched by the scanning signals to drive the associated liquid crystal display elements with the driving signals.
Thus, the picture defects caused by the fact that the picture elements are composed of single TFT per element can be eliminated effectively. However, the improved liquid crystal display panel is still incomplete and has a disadvantage such that if one wants to display an oblique line or a curved line on the display panel, an echelon image is formed because of the arrangement of the picture elements in the matrix.
Explaining this point in more detail, the conventional display panel is composed of substantially simple matrix arrangement, that is, two neighboring picture elements have a common signal or scanning line although a picture element is divided into four pieces and has four display electrodes. Therefore, it is impossible to obtain a continuous oblique or curved line and only an echelon oblique line or curved line is obtained. Furthermore, it is also difficult to reduce the number of picture defects due to the disconnection of scanning or signal lines.
Furthermore, such problems become more significant as the size of the display panel becomes larger and this kind of display panel cannot be adapted for display means of graphs and characters which have greatly been interested in and need a large display area. While it is quite difficult to produce a large number of picture elements (large area display panel) without defects.